Typical hybrid vehicles are equipped with an engine and a electric motor as propulsion sources so as to allow a changeover between an electric drive mode (EV mode, motor drive mode) in which the vehicle is propelled only by the electric motor and a hybrid drive mode (HRV mode, combination drive mode) in which the vehicle is propelled by both the engine and the electric motor. In order to switch between the electric drive mode and the hybrid drive mode, various hybrid vehicle mode changeover control devices have been proposed.
For example, Japanese Laid-Open Patent Publication No. 6-48190 discloses a mode changeover control device that causes a changeover to the HEV mode from the EV mode when the accelerator opening exceeds an EV-to-HEV mode changeover judgment threshold level. When the vehicle is operating in the HEV mode, the mode changeover control device causes a changeover to the EV mode when the accelerator opening decreases below an HEV-to-EV mode changeover judgment threshold level. In order to prevent frequent changeovers between the EV mode and the HEV mode, the EV-to-HEV mode changeover judgment level is set higher than HEV-to-HV mode changeover judgment level so as to provide hysteresis between these judgment threshold levels.